1. Field of the invention
The present invention relates to a process for producing iron oxide red pigments of pure color and to these pigments and the use thereof.
2. Description of the prior art
Processes for producing red iron oxide pigments have been published in: T. C. Patton, Pigment Handbook, Vol.1, New York 1988. In addition, in: SHEN, Qing; SUN, Fengzhi; Wujiyan Gongye 1997, (6), 5-6 (CH), Wujiyan Gongye Bianjib, (CA 128:218378n) a process is reported in which dilute nitric acid acts on elemental iron at elevated temperature, whereby a suspension of haematite nuclei is formed. It is known how to transform such a suspension into a suspension of red pigment and recover the pigment. However, the red pigments produced by this process have a comparatively low color saturation and for that reason are used principally in the building materials industry.
Accordingly, an object of the present invention is to provide improved iron oxide red pigments which have the advantage of the simple method of producing nuclei described by Shen et al., but not the disadvantage of the inadequate color purity.
It has now been found that the relatively high demands of the market on iron oxide red pigments are met by maintaining the reaction temperature of the nucleus formation not at 86xc2x0 C., as recommended by Shen et al., but at above 90xc2x0 C.
The present invention relates to a process for producing iron oxide red pigments, by suspension of haematite nuclei by reaction between dilute nitric acid and elemental iron above 90xc2x0 C., converting the suspension of haematite nuclei to a suspension of haematite pigment and optionally recovering the pigment as a solid.
The present invention further relates to pigments having a color saturation (C*) of more than 39 CIELAB units in the color testing.
The invention also relates to the use of these pigments for the coloring of concrete, plastics, paints, lacquers, and substrates thus colored or coated.
In a preferred embodiment of the process according to the invention, in order to prepare the suspension of nuclei
a) elemental iron is placed in water at a reaction temperature of 90xc2x0 C. to 99xc2x0 C., particularly preferably 95xc2x0 C. to 99xc2x0 C. In another preferred embodiment of the process according to the invention,
b) elemental iron is reacted with nitric acid having a concentration of 2 to 6 wt. % HNO3, In another preferred embodiment of the process according to the invention iron and nitric acid are reacted,
c) in a molar ratio Fe: HNO3 of 1.5 to 16.
This results in a red suspension of nuclei. The suspended particles are shown by an X-ray phase analysis to consist solely of haematite.
The elemental iron is preferably scrap iron, particularly preferably in the form of sheets.
The conversion process can further contain the step of transforming the suspension of haematite nuclei into a suspension of haematite pigment by reaction with iron(II) salt and alkali metal hydroxide and exposure to oxygen-containing gases.
The conversion process can further contain the step of transforming the suspension of haematite nuclei into a suspension of haematite pigment with elemental iron, iron(II) salt and oxygen-containing gases
The pigment formation can advantageously be carried out in two ways. Either metallic iron and iron(II) salt are added to the suspension of nuclei, the whole mixture is heated to 70xc2x0 C. to 100xc2x0 C., preferably 75xc2x0 C. to 90xc2x0 C. and oxidized with 0.2 to 50 l air per hour and per liter of suspension until the required hue is attained, or the suspension of nuclei is heated to 70xc2x0 C. to 100xc2x0 C., preferably 75xc2x0 C. to 90xc2x0 C., iron(II) salt and alkaline solution are added and the whole mixture is oxidized with 1 to 400 l air per hour and per liter of suspension until the required hue is attained.
After the brine has been separated off, the red paste can be transformed into a slurry or dried and ground to form a pulverulent pigment. Soft iron oxide pigments of pure color are obtained. Alpha-Fe2O3 is identified by X-ray phase analysis (ASTM No.: 33.0664).
Preferably, the iron oxide red pigments have a color saturation C* in the color testing of more than 40 CIELAB units.
Preferably concrete and plastics extrudates and automobile lacquers are colored with the pigments according to the invention. Plastics extrudates, according to the invention, also include toners used for electromechanical printing processes and colored with these pigments; the paints include emulsion paints and inks for ink-jet printers.
The pigments according to the invention can be granulated in known manner and applied in this form.
The color testing of the pigments for applications in the field of coatings is carried out in an alkyd resin with 63% linseed oil and 23% phthalic anhydride, (Alkydal L 64, Bayer AG, Leverkusen, DE) to which 5% Luvotix HT additive (Lehmann and Voss) is added in order to increase the viscosity. The volume concentration of the pigment is 10%.
The CIELAB data (DIN 6174) are determined using an Ulbricht globe photometer (illumination conditions d/8xc2x0, standard illuminant C/2xc2x0) including the surface reflection. The color saturation (C*) is regarded as a measure of the color purity of the pigments.